Transcript

1.
Robust File Replication
PPDG Focus Meeting, January 10th 2002
PPDG-11 V0.4
Robust File Replication...........................................................................................................................................................1
1 Introduction............................................................................................................................................................................1
2 Summary of Presentations....................................................................................................................................................2
2.1 JLAB..............................................................................................................................................................................2
2.2 SRB................................................................................................................................................................................2
2.3 Globus (Giggle/Grin).....................................................................................................................................................3
2.4 GDMP(CMS)..................................................................................................................................................................4
2.5 MAGDA(Atlas) ..............................................................................................................................................................4
2.6 SAM(D0)........................................................................................................................................................................5
2.7 STAR.............................................................................................................................................................................6
2.8 Babar.............................................................................................................................................................................6
2.9 Related Work - Condor..................................................................................................................................................6
3 Summary of Discussion Sessions.........................................................................................................................................6
3.1 Interfaces to Robust File Replication services:............................................................................................................6
3.1.1 Redirection Proposal from BaBar..........................................................................................................................7
3.2 Errors, Status, Error Handling, Reliability.....................................................................................................................8
3.2.1 SAM........................................................................................................................................................................8
3.3 Interfaces on which Replication depends ....................................................................................................................8
4 Results of and Proposals from the Meeting..........................................................................................................................9
4.1 Acceptance of Documents:............................................................................................................................................9
4.2 Statements of Direction:................................................................................................................................................9
4.3 Action Items:................................................................................................................................................................10
5 Architecture Diagrams ........................................................................................................................................................11
5.1 SRB .............................................................................................................................................................................11
5.2 SAM.............................................................................................................................................................................12
5.3 JLAB............................................................................................................................................................................12
5.4 WP2.............................................................................................................................................................................13
5.5 ATLAS..........................................................................................................................................................................13
6 Appendix..............................................................................................................................................................................14
6.1 Jan 10th Meeting Agenda ...........................................................................................................................................14
6.2 Jan 10th Participants...................................................................................................................................................14
1 Introduction
The Particle Physics Data Grid SciDAC Collaboratory Pilot includes as one of its core work areas “Robust data
movement and replication” (CS-5 and CS-6). The four participating Computer Science groups are developing Grid
middleware to address components or integrated solutions for these services. The six experiments are deploying file
replication services into production – starting from the use of generic FTP, through initial parallel-stream FTPs such as
bbFTP, gsiFTP, using catalogs of varying sophistication to track and manage the distributed file sets, and experiment
specific higher level components to accomplish end-to-end applications which users can invoke and with which end
users, developers and integrators can interact. PPDG sponsors these groups to integrate and deploy their replication
applications and to share functionality and performance requirements, experience and plans. PPDG then acts to promote
common components and interfaces, consistency and interoperability of appropriate middleware and standards.
This PPDG report is the result of a one-day focus meeting on “Robust File Replication”. Appendix 1 gives the agenda of
and attendees at the meeting. The meeting reflected accomplishments from a lot of work on the part of all the
participating groups. There was a clear interest and preparedness to discuss across the groups future work, technical
and practical issues and directions.
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This report is in 2 sections. It attempts to capture key points of the application and technology presentations to provide
the background to identify a list of future activities of the project, and necessary and relevant areas of work which would
benefit from future discussion. The report does not include the information presented in the talks at the meeting. The
reader is referred to the slides and documents posted off of the meeting agenda page for more detailed information.
http://www.ppdg.net/mtgs/10jan-02/agenda.htm
2 Summary of Presentations
2.1 JLAB
All software based on Web services.
Replica Deep name tree. Translation from GFN to SURL – Global File Name and
Catalog Site-url – host name of the site + url (which includes the protocol and the site).
This is a logical string which can be “redirected” to the actual physical site.
Intent is that naming semantics is links and collections.
Rejected globus replica catalog because it does not support deep trees. Not a
challenging database design to do this. Using mySQL. Performance not an
issue because can improve the h/w. End to end locking and transaction rate.
HRM Listener Application Glue between local site and global replica catalog. Listening to local HRM
level Agent actions and informs other services. A planner? An information server? Each
storage system has an HRM listener. HRM is part of the VO.
Where are there one to many? Switch from Grid to VO this needs more
thought. Wrapper on Jasmine. Soap + mysql
Replication handles requests to make replicas at a higher level than the replica catalog.
Service Handles space requests etc. File Client does not do this. File Transfer Service
does not manage space. Who does?
1. Recommend that a definition of web accessible services should be included in the PPDG Architecture. Is this a
minority opinion? Globus has stated it is a direction they are moving in. The next generation of replica catalog
is defined to be a web service.
2. How should PPDG be defining the web services interfaces? SRB will work with JLAB. Mapping between the
representations can be easily done. How does one define the meaning of the schema? Agree on a minimum
set? Can this be done as a joint effort or is it several parallel efforts. All results should be posted to the PPDG
web sites and comment on it as it is in progress rather than a “final draft for review”. Draft of JLAB
implementation is posted to the meeting web page.
3. GridFTP interaction with Storage Resource managers? Need a discussion with Ian, Carl and Arie. SRM
document addresses some of the issues.
4. Need to communicate error information back through the services and/or layers?
2.2 SRB
SRB Enhancements for BaBar:
SRB->HPSS Driver Glue Connect metadata in SRB DB to HPSS files
SRB Server Extend Extension to use new driver to HPSS and make server support SLAC
Common
Middleware
Remote Proxy Glue Access to and bundling of file transfers.
(DataCutter)
User Client BaBar
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Replication Services
Logical Name Space Replication is a capability “in the logical name space”. Replication integrated
into SRB system. Locking done with timeouts. Inconsistencies can occur.
Registration of Digital Files, Blobs, Database command sequences, URLs. Can see information
Objects from different databases.
Aggregation Container replication; synchronization; staging. Can have a Container that
represents a whole site.
Replica Creation Synchronous, Asynchronous – out of band. (from PPDG requirements)
Replica Access Automated fail over to alternate copy
Latency Management
Data Transport
Meta Data Transport
1. Remote Proxy – possibility that will need scheduling service, and mechanism for improving efficiency of
file transfers.
2. Need to access metadata independent of file access. Need to provide bulk metadata import and
registration. Discovery based on attributes.
3. Storage System access and data transport interface are site specific.
4. Any thoughts on linking the BaBar metadata catalogs Oracle and Objectivity ? Complex but has been
done with Objectstore.
5. Asynchronous replica creation (k out of n is a success). using background service was not requested by
BaBar nor implemented. by SRB. Relation to partial result? Could benefit from more discussion.
6. Architecture
a. Storage Abstraction – is this /should this be a common components? How does it relate to the
HRM definition? Includes latency management.
b. Catalog Abstraction
2.3 Globus (Giggle/Grin)
First version of Replica Catalog and Management Services is in production as part of Globus V2.0 and integrated into
GDMP and EDG TestBed 1. The comments relate to the developments of the new components : Replica Location
Service (RLS) which augments the Replica Catalog, and Reliable File Transfer Service which is a component above the
File Transfer layer. The first prototype implementation of the RLS is scheduled for 4/02 and a production version for
integration with EDG TestBed 2 in 9/02.
Replica Catalog File attributes are kept in meta-data catalog which is outside the domain of the
Globus service?
Reliable Replication Combine storage system operations with replica catalog updates.
Replica Selection Estimate performance
Relies on Information Services
Replica Location To an end user the functionality will appear as equivalent to the set of Replica
Service Catalog, Replica Selection and Replication Managament
Framework:
Reliable Local State
Global State with
Relaxed Consistency
Reliable File Reliable transfer of byte streams. Built on top of GridFTP.
Transfer Service http://www.mcs.anl.gov/~madduri/RFT.html
Reliable Replication Reliable Replication Service. Who is responsible for establishing the reliability,
Service verifying and determinine the Catalog consistency. Catalogs within RLS include the
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Storage System catalog.
1. New implementation of Replica Catalog supports logical files in several collections and containers?)
2. Name Space. Could one map to the UNIX file system name space? Is this something that PPDG wants to
input to? WP2: Does one need to define Name Space semantics? Is the definition of database tables
sufficient – ie arbitrary set of attributes that defines a name?
3. “Collection” use overload.
a. Container/Aggregation. Same as a data object. Clusters.
b. Selection Set/Collections. Logical organization.
These are orthogonal.
4. Could Globus interface discuss with JLAB and SRB before completing the definition of the interfaces for
RLS?
5. Difference between Replica Management and RLS was not completely clear?
6. Impact on End User of different consistency levels. Should be none except for performance? Depends on
the user API. User gets “probability” that file is in the stated location. This is always true.
a. Does End User gets information that is “Wrong”? - possibly. But this is true given errors that
can occur with completely design which guarantees consistency?
b. Does End User always get correct information but performance is affected? Yes.
7. Semantics of the Hints/Location Service needs to be separate from those of the File Delivery Service.
8. WP2 has seen no performance issues with current version of Replica Catalog.
2.4 GDMP(CMS)
Grid Data Mirroring Package. V2.0 is included in EDG TestBed 1 and V2.x will be in VDT 1.0.
Publish/Subscription GDMP Local catalogs – text files - keep lists and state.
Manager
Replica Catalog Globus Updated when replica “pulled”. Can be used as a push model with the GDMP layer
File Copier GridFTP Interfaces to the Storage System
Storage System Looking at the HRM. How does the interaction happen?
Interface
Replica Optimizer WP2 Being designed. Is this a potentially “common component”. Workshop is at CERN
week of Mar 15th
1. GDMP works on Containers as well as single files. This is an enhancement to the Globus Replica catalog/
management.
2. Error recovery use cases.
a. May republish a file that already succeeded. Globus replica catalog refuses duplicate entry of
logical file.
b. May be knowledge in the catalog you don’t know about. Should protocol include a Transaction
Index and 2 phase commit?
c. Where is the responsibility to determine validity of catalog?
d. Is GDMP functionality replaced by Globus Reliable Location Service in the future? Not
completely. Will need the Publish/Subscribe layer.
2.5 MAGDA(Atlas)
MAGDA is being used and further developed by ATLAS as a vertically integrated framework available for testing,
experiment development and production use. Gsiftp and scp are used for the file copy, mysql as the database. To date
other components are ATLAS developed.
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Logical File Name Supports collections and container. Arbitray string. Name is unique in a VO, includes
Space Replica Number.
File Catalog Mysql database. Mysql accelerator written by ATLAS for sets of database updates.
Replica catalog loader written but not tested. No transaction locking to date.
Storage System Data repository. Site + Location. Host can access a set of sites.
File Discovery Agent Spider finds files and registers them
Replication Service Replication Operation done by tasks. (Data Placement Jobs). Master Instance is a
requirement – addresses consistency issue. Use scp/gsiftp. Gdmp integration
underway. Cost of access – only allow access from local cache and site. Automated
optional delete of replica.
User Web Interface Web pages for requests and status
1. Consistency maintenance – Assured Current.
2. Trusted Files. Supports new versions of files which must be published. Can one rephrase this?
3. HEMP – Hybrid Event Store Metadata Prototype. Related to Data Signature work.
4. Replication Jobs. Data Movement scheduling needs a fuller discussion.
GDMP Issues:
1. One root disk directory per site
2. Subscription updates bring in all new data for a site
3. File collections not used
4. LFN fixed as ‘dir/filename’ (RC constraint)
5. Doesn’t catalog or directly manage files in MSS
6. Wr
7. ite
8. access to tmp, etc disk areas required for all GDMP users
7. System state info (in files) only available locally
General discussion topics:
1. Policies for Storage and Access.
2. User view of MAGDA? Similarity of services with SAM and BaBar needs?
2.6 SAM(D0)
SAM is in production use by D0 as an integrated data grid system. The file handling, replication, routing services were
developed some time ago. The presentation focused on some of the robustness features in the file copying components
and deployment of the integrated distributed system – it is not a complete view.
Failover If error from one replica automatically fail over to another
Cleanup Release resources if task or job fails. Detection of abandoned jobs.
Responses to Errors Timeout if resources held too long without action.
Node error results in rerouting of the data to healthy nodes
Exit handler in User process which calls DH system
Resilience Automatic restart of servers and jobs. Retries of replication. Separate movement of
data itself from that of the metadata to separate dependence on storage system and
data catalogs.
Performance Tuning Parallelize database access layer.
Integration Features Validation agents. Error message translation and interpretation at Component
Interfaces. Tunable timeouts at every interface. (No checksums.)
1. Timeouts as an error mechanism. Pluses and minuses.
2. Unexpected/incorrect behaviour of layers depending on (e.g. file copier) takes a lot of time and work to
code for/around.
3. Complete logs help debugging and diagnosis.
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2.7 STAR
STAR is working with the SRM project on the integration of the HRM implementation of the SRM standard in an end –to-
end application.
Replica Catalog mysql
File transfer Globus GridFTP
Storage Management SRM SRM-HRM. Retries work when there is a storage system error.
2.8 Babar
BaBar has a prototype of database replication using the SRB replication services. This prototype is being modified to
separate the catalog information in MCAT - leaving the core replication schema in MCAT and the BaBar extensions in
another DB.
2.9 Related Work - Condor
Condor developments were not reported in the meeting, are related to the topics at hand and are candidates for PPDG
work: Nest http://www.nestproject.org/ , ftp-lite http://www.cs.wisc.edu/condor/ftp_lite and the pluggable file
system http://www.cs.wisc.edu/condor/pfs and kangaroo http://www.cs.wisc.edu/condor/kangaroo
3 Summary of Discussion Sessions
These notes are from the scheduled and impromtu discussion sessions. As such they are incomplete and reflect
periods of time when the notetakers were otherwise engaged.
A JOB is a schedulable unit or a schedulable transaction.
3.1 Interfaces to Robust File Replication services:
MAGDA, Globus, SRB, SAM –
Web Services for this uniformity? Or Protocol Question – commands and/or attributes that are included.
Do we want to retrofit and/or wrap existing systems with the same interface definition but different
implementation.
Are there separate services for Replica Catalog interface and/or Replica Services.
Semantics of replica systems.
Assume live in a heterogeneous world and one implementation can talk to another implementation. May
require reimplementation.
EDG is not trying to solve the problems “of the whole world”. Bottom up approach and identify components.
Core set of capabilities.
For JLAB Publish/Subscribe is a Replica Policy.
Low level API for file transfer should not be dependent on whether being used in Replication or not.
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Where does bulk transfer of data – container of containers. Is this a separate concept or not? Does it affect the
semantics and model of consumption of the data. Is there lazy consumption or not? Where do the policy and
planning interfaces occur? Can a file be regarded as a container and it is then decomposed and partially copied
– this is a task for SRB ASCI project.
How high up the service layers are we going to go? What are the collective and application level components.
Do we want/need to address the end user layers?
With reference to DGRA V2.09
User Interface
Replica Management 9.1 register, move, copy
Replica Catalog Service 7.3 “catalog-only” requests and
collection definition
Local Replica Catalog 5.5.2
Storage Resource (system, 5.1 storage requests and information
element)
Reliable Transfer 9.1 copy only requests
Publish/Subscribe
Is there a consistency mechanism as part of the API? Validation and transaction API? What is the semantic for
this?
How to address fact the “place to memory’ and ‘place to disk’ can have same semantics but are certainly not
replaceable and are not necessarily interoperable.
Need to discuss the State of the file and as well as the Status of the replication and file storage/copy.
Coupling between Storage Element and Virtual Storage Element or Replica Catalog. Need to be careful about
wanting a full file system semantics of a unix file system.
Are people prepared to get together to work out the overlap and commonality between current
implementations. Then deliver this to PPDG. Should not take more than 2 months. Not clear what benefit this
would have – we have representatives of all the implementations available to review any common proposals.
RLS. Local Catalog in next week or 2. Index Node specification – prototype version by the end of March.
Globus Replica Management API:
http://www-unix.globus.org/api/c/globus_replica_management/html/index.html
3.1.1 Redirection Proposal from BaBar
The BaBar redirection requirement and implementation proposal is posted off of the agenda web page. It has
been previously discussed in PPDG meetings and was revisited here in light of the next round of Globus/WP2
design and implementation work:
1. Redirection is part of the WP2 design for TestBed 2.
2. RLS allows a first level of indirection. Need to leave protocol open to allow later addition of this redirection
capability. This is has been agreed to for a while, but needs detailed implementation details.
3. For web services interface – redirection is explicit in that there is a 2 step process for accessing the byte
stream in the SRM document.
4. Manual lookups – always doing a redirection.
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Agreed that this issue is being addressed and the next discussion should be to review the implementation after
the first prototype version of RLS is released.
3.2 Errors, Status, Error Handling, Reliability
Discussion was driven by the slides posted of the agenda web page.
Should one provide a layer that takes all error information and interprets it. Can design a “perfect error system”
will always have to translate the information for some other component.
Strings vs Error Codes – give the Details or the Essence. Maximum length of string to have user read it. So
“Summary String” and “Detailed String”.
Need to address Status from success as well as failure e.g retries.
What is in the error and status handling that is better in the information/monitoring system?
Diagnosis and response can/should/is an independent activity? Who uses the information for what –
debugging , diagnosis, human response.
PPDG should decide what we want to do about Error Handling? Agreement that this is an important area
which always takes much work for end-to-end application and distributed system integration and deployment.
Server Process and/or Service Machine died in the middle of a catalog/database update. Details are different
although report to the user is the same.
Should system be robust to system administrator deleting a logical file somewhere. In Giggle can make sure
local catalog and local storage are consistent. This might be too costly? What happens if one loses a file?
Status e.g. how many retries, automated failover information, of successful operations also important.
Definition of file STATEs part of overall understanding of errror, status, consistency, robustness issues.
3.2.1 SAM
SAM status blocks were not included to date in the presentation. SAM keeps a nested stack of errors and
structures. All the information is contained in the structure. Ultimately printed as text. http://d0db.fnal.gov/sam/
doc/design/status.html , http://www.ppdg.net/mtgs/10jan-02/SAMErrorCode.idl.txt ,
http://www.ppdg.net/mtgs/10jan-02/SAM_Status.idl.txt . Examples:
>>>>>> Starting project with the Station % CERR 11-Sep-2001 15:57:02 SAMManager:sammgr -
M aster %ERLOG-w SAM: PROJECT MASTER:
Defaulting to quot;newquot; dataset version Project master error caught in SAMManager::locatePM()!
CORBA Exception, station is probably Error message: Project master unreachable!
dead (Minor: 0 Contact sam-users@fnal.gov!
Completed: COMPLETED_NO) sammgr 11-Sep-2001 15:57:02 SAMManager:sammgr -
SAMManager:sammgr Waiting for the project master (no timeout).
%ERLOG-e UNKNOWN:
CorbaUtil::Resolve:
'/SAMStations/central-analysis/09_11_01_15_56:Project' not found
3.3 Interfaces on which Replication depends
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The Data Grid Capabilities document (PPDG-8) was used as a basis for discussion. This document will be recast
into categories to map onto DGRA and MAGDA will be included.
Latency management – what are the technical details.
Robustness – capabilities not in common.
Asynchrony support
Consistency state.
Logical File Names:
1. Does Unix semantics Logical File Names follow through into functionality e.g. ACL for directory affects
ability to create new files. How does Authorization get affected? Is this part of the architecture/design.
Multi-part authorization process.
2. How does one do a Physics Meta-Data Query. Logical name space attributes or the name?
3. Are Names of Files “meaningful” or are the “strings that identify a set of meta-data”.
4. Does update of a file create a new entry? Should version be part of the significant name?
4 Results of and Proposals from the Meeting
4.1 Acceptance of Documents:
The following PPDG documents were accepted. Comments, changes, new versions are anticipated. These documents
are PPDG project document deliverables in Common Services CS-7.
PPDG-10 Numeric Requirements for the Replica Catalog Service V0.2
PPDG-9 Common Storage Resource Manager Operations V1.0
PPDG-8 Data Grid Implementations - Comparison of Capabilities, V6
This paper is proposed to be PPDG-11 - Robust File Replication, PPDG Focus Meeting Report
4.2 Statements of Direction:
There has clearly been a lot of progress in the design, implementation and deploying of Replication Services in the PPDG
experiments over the past year. Successes include:
a. End to end application tests by all experiments.
b. Delivery and prototype use of new Globus Replication services and extension of SRB and HRM
common services.
c. Accepted common terminology and use of Data Grid Reference Architecture definitions.
d. Documenting performance requirements and system capabilities.
e. Progress on more detailed interface, architecture and protocol definitions
f. Inter-team discussions on new designs and interfaces.
PPDG will continue to collaborate with EDG on GDMP in its developments for WP2 TestBed 2 and integration with
Giggle. Ppdg-exec should discuss this with EDG/WP2, CMS and Globus as part of PPDG Year 2 planning.
a. Need to define which pieces to leave as GDMP specific layer. Is GDMP still a “CMS specific”
PPDG project activity? For EDG it is not CMS specific.
b. Need to address the issue of GDMP V2 support as V3 is developed and deployed.
.
JLAB/SRB Project Activity service specification will be the nascent protocol definition for Replica Management for PPDG
review/input and adoption. This is a possible discussion topic for the Feb PPDG collaboration meeting if there is time. It is
possible that Globus might be able to consider contributing to and/or reviewing this.
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While there is continued concern at multiple implementations in experiments of file transfer and replica management it is
clear that during this phase of the project it is most constructive to be exploring different ideas and directions as a
precursor to moving towards more commonality. We expect continued discussion of this issue.
There is still significant work to be done to have a Robust File Replication system that meets the needs of all the PPDG
application groups.
4.3 Action Items:
SRB/JLAB interface to Storage Element and JLAB,SRB document first draft 2/20/02
Replication Management (web service definition)
GridFTP interaction with Storage Resource ppdg-exec phone con with Carl, Ian,
managers Bill, Arie to initiate the discussion
Container and Collection consistency in use ppdg-exec review PPDG documents 2/20/02
GDMP and RLS issues (ATLAS GDMP issues, Ppdg—exec phone con with GDMP, 2/20/02
PPDG Year 2 planning, Master Replica. WP2, CMS, ATLAS, Globus, Andy
Review next version of Globus Replication Agenda of PPDG phone con. 1/30/02
development
Review Local Replica Catalog Interface Agenda of PPDG phone con – AC, 1/30/02
SM
Data movement scheduling Agenda of PPDG phone con Before
4/02
Error Reporting, Handling and Response in the 2 page paper from ppdg-exec. 2/20/02
PPDG Environment Agenda of PPDG phone con 3.02
Revisit outcomes Another focus meeting Decide in
April.
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